The Universal Serial Bus (USB) specification defines a communications architecture for connecting a host computer with peripheral devices. Many electronic devices use USB interfaces, such as wireless networking devices, modems, scanners, speakers, portable or external media storage devices (disk drives, music players, optical drives, etcetera), monitors, keyboards, joysticks, and printers. These USB devices are categorized into various device classes including display devices, Human Interface Devices (HIDs), audio devices, mass storage devices, and communication devices.
The USB specification allows only 127 devices to be attached to the same USB host. This 127 device limit also includes hubs. Thus, for practical purposes, only around 100 USB peripheral devices can be attached to the same USB host. The 127-device limit becomes a constraint in some USB applications.
For example, hundreds of devices may need to be tested by the same computer at the same time. It may be desirable to test these devices through USB connections. However, the restricted number of devices that can be simultaneously connected to the USB host limits the viability of using USB in certain test and measurement applications.